1. Field of the Invention
The present invention relates to alkali cellulose and a method for preparing cellulose ether using the same.
2. Description of the Related Art
For preparation of cellulose ether, known is a method comprising steps of bringing highly purified pulp into contact with an alkali solution to prepare alkali cellulose, and then etherifying the alkali cellulose with an etherifying agent. Although the cellulose ether obtained as a final product becomes soluble in water by properly controlling the degree of substitution, the cellulose ether contains a water-insoluble portion so that the light transmittance of the aqueous solution may be lowered or the commodity value of the cellulose ether may be damaged because the water-insoluble portion is considered as a foreign matter.
The insoluble portion is caused by the presence of a portion having a low degree of substitution which does not have enough substituents to permit dissolution of the cellulose ether in water. One of the causes is uneven alkali distribution in the alkali cellulose.
The rolls the alkali plays include (a) swelling the cellulose to change the crystal structure in the pulp, thereby accelerating the penetration of the etherifying agent, (b) catalyzing the etherification reaction of an alkylene oxide, and (c) serving as a reactant for an alkyl halide. A portion of the pulp out of contact with the aqueous alkali solution is not involved in the reaction and therefore remains as an insoluble portion. Thus, uniformity of the alkali cellulose is related to an amount of the insoluble portion in the alkali cellulose.
A method which has been employed widely for the preparation of alkali cellulose includes one described in Japanese Patent Application Examined Publication No. 60-50801/1985 or Japanese Patent Application Unexamined Publication No. 56-2302/1981 in which an alkali is added in an amount necessary for etherification to pulp powder obtained by pulverization of pulp, and then mixed mechanically. In this method, however, the alkali is not distributed to all parts of the pulp powder so that some parts of the pulp power remain out of contact with the alkali. As a result, some parts fail to become cellulose ether, stay in the product as an unreacted substance and deteriorate the quality of the cellulose ether. Thus, this method causes a problem.
In order to avoid causing such a problem, a method comprising steps of impregnating a sheet of pulp in an excess alkali solution to allow the pulp to absorb a sufficient amount of an alkali and then pressing the impregnated sheet to remove an unnecessary portion of the alkali and control the alkali to a predetermined amount. When this method is performed industrially, it is the common practice that a rolled pulp is allowed to be rotated freely by elevating the rolled pulp by a supporting shaft put through the central pipe of the roll so as to bring it up from a floor surface, or placing the rolled pulp over a roller. Then, a sheet is drawn from the rolled pulp and introduced into an immersion tank. However, according to this method, the operation is often interrupted because the pulp sheet is torn by a pull force during the immersion. Moreover, to achieve mass production, a huge immersion tank is required for immersing the pulp sheet therein for a predetermined time. Accordingly, this method has defects such as necessity of an adequate space for the tank and inevitable rise in the investment cost. On the other hand, when pulp chips are employed, the cake of them has an irregular surface so that the uneven squeeze occurs when pressed by the press machine. Uneven alkali distribution caused by the uneven squeeze deteriorates the quality of the alkali cellulose.
In page 433 of “Encyclopedia of Cellulose” edited by The Cellulose Society of Japan (published on Nov. 10, 2000), described is a preparation step of alkali cellulose for preparation of viscose, the step comprising adding pulp to an alkali solution to form a gruel-like slurry and squeezing the slurry by a slurry press. Although the defects caused by use of a sheet of pulp are solved, the slurry press causes uneven squeeze and thereby deteriorates the quality of the alkali cellulose due to uneven alkali distribution. In addition, it is difficult to obtain alkali cellulose having a relatively small alkali content which is required as a raw material of cellulose ether using this method alone because of the limitation of the squeeze performance. Thus, it is difficult to apply this method to the preparation of cellulose ether.
In Japanese Patent Application Examined Publication No. 3-73562/1991, described is a method for preparing alkali cellulose having a desired composition, comprising steps of preparing alkali cellulose from cellulose and excess alkali, and then washing the alkali cellulose with a hydrophilic solvent for removal of the alkali. However, this method needs huge equipment and many operations. In addition, the hydrophilic solvent remains in the alkali cellulose and causes a side reaction with an etherifying agent. Accordingly, a reaction efficiency of the etherifying agent is reduced. Neutralization of the washing liquid or recovery of the alkali is required. Thus, this method is industrially difficult.
According to the preparation method described in International Patent Application Japanese-Phase Publication No. 2001-518127, alkali cellulose having 0.316 to 0.643 of weight ratio of alkali metal hydroxide/cellulose is obtained as a raw material for preparation of viscose by dispersing a cellulose-containing material in an aqueous solution of sodium hydroxide and then subjecting the resulting dispersion to centrifugal separation. As described in the publication, only an aqueous solution of sodium hydroxide having a concentration of 14 to 22% by weight is usable in the method. The reason is as follows. Immersion time for the cellulose-containing material in the aqueous solution of sodium hydroxide during the dispersion of the material in the solution is not controlled or adjusted. Accordingly, the cellulose-containing material continues to absorb the aqueous sodium hydroxide solution until the absorption reaches almost a saturated amount. An aqueous solution of sodium hydroxide having a considerably low concentration must be employed in order to obtain alkali cellulose having a desired weight ratio of alkali metal hydroxide/cellulose. However, alkali cellulose prepared using an aqueous sodium hydroxide solution having such a low concentration is economically disadvantageous because a side reaction between an etherifying agent and water occurs in the subsequent step for preparing cellulose ether. In addition, the cellulose ether thus obtained cannot have a desired degree of substitution. Moreover, an aqueous solution of the cellulose ether thus prepared is inferior in transparency.
The utilization ratio of the etherifying agent correlates with the water content of the alkali cellulose. As the water content is smaller, the utilization ratio increases, meaning that a desired degree of substitution can be attained by a small amount of the etherifying agent.
As a preparation method of an alkali cellulose having a small water content, known is a method comprising suspending finely pulverized cellulose in an inert solvent, carrying out first-stage addition of sodium hydroxide in the solid form to partially swell the cellulose with the organic solvent in advance, and then carrying out second-stage addition of 40 to 60% sodium hydroxide solution (Japanese Patent Application Unexamined Publication No. 56-145901/1981). However, the cellulose ether prepared using the alkali cellulose obtained by the method is inferior in the transparency of its solution. It is also unsatisfactory from the economical viewpoint because sodium hydroxide in the solid form is expensive.
It is also known that alkali cellulose having a small water content is available by alkalizing cellulose with sodium hydroxide and washing the alkali cellulose with a proper solvent prior to an etherification reaction (Japanese Patent Application Unexamined Publication No. 58-196202/1983). During washing, however, not only water but also sodium hydroxide is washed away so that only methyl cellulose having a low degree of substitution is produced.
When preparation of an alkali cellulose is started using an aqueous sodium hydroxide solution having a high concentration, a method comprising steps of adding the alkali in an amount necessary for the etherification reaction to pulp powder obtained by pulverization of pulp and then mechanically mixing them as described in Japanese Patent Application Unexamined Publication Nos. 53-41356/1978 or 56-2302/1981 cannot uniformly distribute the alkali in the pulp powder. Accordingly, some portions of the pulp do not contact with the alkali so that they remain as they are without becoming cellulose ether. They are present as an unreacted substance in the product. Thus, there is a problem of poor quality of cellulose ether.
In a method comprising steps of immersing a pulp sheet in an excess alkali solution to allow the pulp to absorb a sufficient amount of the alkali and then pressing the sheet to remove an excess alkali for adjusting the alkali content to a predetermined level as described in Japanese Patent Application Unexamined Publication No. 49-61272/1974, when an aqueous sodium hydroxide solution having a high concentration is used, a solid matter (sodium hydroxide hydrate) precipitates from the aqueous sodium hydroxide solution unless it is maintained at a considerably high temperature. There is a problem of apparatus troubles caused by the precipitation.